Red blood cells (RBCs) are specialized cells whose primary role is to deliver oxygen. Approximately 95% of protein in RBCs is hemoglobin, which is the iron-containing protein that binds oxygen in the lungs, and releases it in the rest of the body where it is needed. RBCs obtained from donated blood are essential to modern medical care. Nearly 15 million blood units are transfused every year in U.S. and about 85 units million globally.
While donor blood is important, like any other intervention, it has side effects and limits to its utility. First, blood units contain residual donor antibodies (i.e., alloantibodies) that can increase inflammation and worsen outcomes in recipients. Second, RBCs inside the body have a lifespan of about 120 days, during which they are constantly metabolizing nutrients and releasing waste products that the kidneys, liver, and spleen filter. In a donor unit. RBCs are still undergoing metabolism and shedding waste products; however, the waste (e.g., hemolysate) accumulates in the donor blood bag. These detrimental substances include acids, potassium, hemoglobin, and iron, which are damaging outside of the RBCs; and pro-coagulant and pro-inflammatory microvesicles or microparticles. The accumulation of these byproducts lend to cell death causing the release of additional harmful substances. Consequently, concern has been raised about the safety of aging blood and the maximum shelf life of blood which is conventionally 42 days after being drawn in the U.S., while even shorter (35 days) in Europe.
Microparticles are fragments of cell membrane that house hemoglobin and pro-coagulatory and pro-inflammatory lipids. Excess potassium in the blood (i.e., hyperkalemia) is a risk for arrhythmias, particularly in sensitive populations, such as newborns. After the immediate life-threatening hyperkalemia and immune-mediated anaphylaxis, free hemoglobin and iron are the next most-recognized components to cause complications during transfusion. Thus, systems have evolved to capture and remove both cell-free hemoglobin and iron. However, the efficacy of these systems varies largely by individual and can be saturated. In fact, neonatal transfusionists commonly wash blood before transfusion into newborns, whose protective systems are under-developed and can be easily saturated, especially in the absence of “fresh” blood, Free hemoglobin rapidly consumes nitric oxide (NO), a molecule with many important roles in maintaining vascular homeostasis. NO is a vasodilator that inhibits platelet aggregation and controls inflammation and mitochondrial function. Thus, free hemoglobin scavenging of NO leads to acutely increased blood pressure, platelet aggregation, and inflammation. Free iron can also be damaging due to numerous iron catalyzing oxidative reactions, which form free radicals and other oxidants that enhance inflammation and cause cell and tissue damage. Additionally, free iron enhances clot formation and bacterial growth.
The above-described mediators of complications, whether antibodies or bioactive byproducts such as hemoglobin or microparticles, are located in the plasma component. Washing cells has been a way to reduce these side effects. However, washing requires centrifugation of cells, removal of plasma/storage solution, and addition of a new buffer, with this process being repeated two or more times. This process requires anywhere from 30 to 120 minutes, which is unacceptable in certain clinical scenarios. Additionally, washing can damage RBCs through repeated centrifugation at or above approximately 1,000 times the force of gravity, as is done clinically, causing additional hemoglobin release.
There are differences between young RBCs and old RBCs. For example, as a RBC ages, it progressively becomes more adhesive, and it loses negatively-charged surface molecules and volume. Transfused young RBCs last in the circulation longer than old RBCs. However, washing donor blood does not distinguish old RBCs from young RBCs. The current minimum for RBCs survival 24 hours after transfusion is approximately 75%. The destruction of old RBCs after transfusion contributes to saturation of the iron-binding capacity of the plasma, resulting in circulating free iron.
Conventional filtration products are available to treat whole blood, blood cells, or blood components. However, these conventional products do not address the specific challenge of improving the quality of packaged red blood cells after prolonged storage in order to make the RBCs more suitable for transfusion. Therefore, there is a need to extend the useful life and quality of donor RBC units, and a faster alternative to washing aged, stored RBCs to remove potentially harmful components from stored donor units without the need for repeated, time-consuming cycles of centrifugation and washing that can also lead to RBC damage.